Our objectives for the coming year are as follows: (1) We hope to complete our autoradiographic studies of the time of arrival of the commissural and ipsilateral afferents to the hippocampus. This work is already at a fairly advanced stage, as indicated in this progress report. (2) We similarly hope to complete a three-dimensional analysis of the growth and development of the granule cells of the dentate gyrus, using a computer-controlled microscope developed in this laboratory. (3) We shall continue the work on the growth and maintenance of hippocampal neurons in vitro, which is an essential preliminary to any serious biochemical attack on the membrane specificities of hippocampal neurons, and shall attempt to clarify some of the parameters of cell survival in this system, to document the rate and nature of dendritic growth, and to begin the analysis of the pyramidal cell surface. (4) Having demonstrated in the rat that the fields of the hippocampus do not project to the hypothalamus (as has been believed for more than a century), we propose to extend this study to the hippocampal formation of the squirrel monkey so that our findings may be more directly relevant to the understanding of the limbic system in the human brain.